STV has long been advocated by political scientists as one of the most attractive electoral systems, but its use for legislative elections has been limited to a few cases—the Republic of Ireland since 1921, Malta since 1947, and once in Estonia in 1990. It is also used for elections to the Australian Federal Senate and in several Australian states, and for European and local elections in Northern Ireland. It has been adopted for local elections in Scotland and in some authorities in New Zealand. It was also chosen as the recommendation of the British Columbia Citizens’ Assembly.
The core principles of the system were independently invented in the 19th century by Thomas Hare in Britain and Carl Andræ in Denmark. STV uses multi-member districts, and voters rank candidates in order of preference on the ballot paper in the same manner as under the Alternative Vote system. In most cases, this preference marking is optional, and voters are not required to rank-order all candidates; if they wish, they can mark only one.
After the total number of first-preference votes are tallied, the count then begins by establishing the quota of votes required for the election of a single candidate. The quota used is normally the Droop quota, calculated by the simple formula:
Quota = (votes / (seats +1)) +1
The result is determined through a series of counts. At the first count, the total number of first-preference votes for each candidate is ascertained. Any candidate who has a number of first preferences greater than or equal to the quota is immediately elected.
In second and subsequent counts, the surplus votes of elected candidates (i.e. those votes above the quota) are redistributed according to the second preferences on the ballot papers. For fairness, all the candidate’s ballot papers can be redistributed, but each at a fractional percentage of one vote, so that the total redistributed vote equals the candidate’s surplus (the Republic of Ireland uses a weighted sample instead of distributing fractions). If a candidate had 100 votes, for example, and their surplus was five votes, then each ballot paper would be redistributed according to its second preference at the value of 1/20th of a vote. After any count, if no candidate has a surplus of votes over the quota, the candidate with the lowest total of votes is eliminated. His or her votes are then redistributed in the next count to the candidates left in the race according to the second and then lower preferences shown. The process of successive counts, after each of which surplus votes are redistributed or a candidate is eliminated, continues until either all the seats for the electoral district are filled by candidates who have received the quota, or the number of candidates left in the count is only one more than the number of seats to be filled, in which case all remaining candidates bar one are elected without receiving a full quota.
